... because they can go off at random times between midnight and 6am, and because it's up high you have a heck of a job working out where it's coming from. I guess it was dust in our ceiling cavity, or a mouse pushing the test button...

I've never heard of anybody ever installing a smoke alarm in a roof space and it's a bad idea for the reason you've discovered - the roof space is full of dust so they're going to trigger continually when dust enters the chamber. Both ionisation and photoelectric alarms will suffer this issue.

We have a central smoke alarm attached to the alarm system between our bedrooms in the hallway. I've put extra smoke alarms in the bedrooms and kitchen so we get earlier warnings if there is a fire. I could get more wired in, I think I have a couple more slots on the alarm available, but because I already got standalone ones which will be a bunch cheaper I bet.

I have always thought that smoke alarms in the ceiling cavity would be a good idea. It is a hot dry dusty place with lots of electrical wiring etc. so is probably an area that fires could start in. Trouble is that those conditions also make it an unsuitable place to install smoke alarms.

I have logged a fraction over 60 in my roofspace at the top underneith the black metal tiles. I would expect that would cook any battery in no time if it was a battery alarm.

Also all the smoke that blows thru there from the neighbours fires would set it off in winter.

In addition: Most heat sensors will go off around 58 degrees Celcius. The smarter ones will look at how quickly the temperature changes, but any fairly cheap heat alarm would probably just alarm when it passes 58 degrees.